(1) Field of the Invention
The present invention relates to pulleys for power transmission belts, and belt power transmission devices.
(2) Background Art
In a power transmission device that uses a flat belt, the flat belt might zigzag during running, or might cause side tracking toward one side of a pulley. This is because, compared with other power transmission belts, a flat belt is susceptible to changes in components of a power transmission device, such as deviation of a pulley shaft from its proper position, deflection of the pulley shaft caused by a variation in shaft load, and wobbling of a pulley. If such zigzagging or side tracking occurs, a flat belt comes into contact with a flange of a flat pulley to scuff a side face of the flat belt or fray a cord.
As the measures to prevent such a problem, in a known power transmission device, a crown (i.e., a crowned curved surface) is formed at a periphery of a flat pulley. However, if importance is attached to the stability of running of the flat belt (i.e., prevention of zigzagging or side tracking) to reduce the radius of curvature of the crown, stresses are concentrated at the center of the belt width. Thus, the entire belt width cannot be effectively utilized for power transmission, which leads to early fatigue of a cord and degradation in power transmission capability.
To cope with this, in another known power transmission device, a large number of grooves are formed at a periphery of a flat pulley so as to be spaced apart in a circumferential direction (e.g., see Japanese Unexamined Patent Publication No. 6-307521). Specifically, these grooves are extended symmetrically from the center of width of the pulley to both sides so as to be V-shaped, and a friction that pulls a flat belt to the center is produced between the flat belt and the pulley, thus preventing zigzagging and/or side tracking of the belt.
However, if the above-described grooves are formed at a flat pulley, the fabrication cost of the flat pulley is increased, and furthermore, it is difficult to certainly prevent zigzagging and/or side tracking of the flat belt by merely forming these grooves. Therefore, the fact is that a flat belt power transmission device is not sufficiently utilized although it does not cause great loss resulting from bending of the belt and has considerably high power transmission efficiency as compared with other belts such as V-belts.